1. Field of the Invention
The invention relates generally to a method and apparatus for determining data sector locations, and more particularly to a method and apparatus for digitally correcting sector pulse placement in a direct access storage device (DASD).
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks.
All DASD units must have a method to position each data head over the proper radial location to write a track and again, to position it very close to the same location to read the track. With the higher level files using a voice coil type of actuator, a feedback mechanism must be provided to locate and stably hold the head on a given track. Typically, track accessing and track following is provided utilizing a magnetically written pattern in the DASD unit. A dedicated servo system employs one surface of one of the disks in the DASD on which to have all the tracking and access information. A sector servo system uses small portions of tracks between each or between several sectors on each track of each data surface to provide the tracking and access information. A hybrid servo system uses both to obtain advantages of each type of servo.
Inherent to a fixed block DASD is the division of the disk into equally sized segments or sectors around the disk surface. Individual sectors contain a predefined size of individual groups of customer data saved for later retrieval and updates.
It is desirable to provide a DASD with variable sized sectors that can be defined to be unique according to the individual data format desired by the customer. Many known DASD units using sector servo are limited to the use of predefined areas of the disk for storing customer data. To provide variable sector sizes, known arrangements typically require analog circuitry for supplying a sector clock locked to the rotational speed of the disk. Locking the sector identification pulses to the rotational speed of the disk is necessary to ensure that the location of each sector is known, without requiring large sector overhead or wasted disk space for sector identification bytes to accommodate for variability of the motor speed over time. Known DASD units using dedicated servo do not easily facilitate the use of a large flexible array of sector sizes.